Projecting images onto spherical and other three-dimensional projection screens presents several issues that bear on the quality of the images. The most significant problem faced by early versions of spherical screens was adequate coverage of the screen surface with an image. For example, systems that used multiple projectors mounted inside a spherical rear projection screen such as the one described in U.S. Pat. No. 3,586,432 provided very limited coverage of the screen and made it difficult to access the projectors for maintenance. Systems using external projectors, such as the kinds described in U.S. Pat. No. 4,427,274 and U.S. Pat. No. 4,859,053, provided better access to the projectors but were either limited to producing an image on a hemispherical screen or required projectors to be located at opposite ends of a sphere. These limitations have been substantially overcome by systems in which the projectors project the image onto one or two convex mirrors within the sphere from which the image is reflected onto the surface of the screen. This approach is well described in U.S. Pat. No. 6,409,351.
Two significant challenges associated with three-dimensional projection systems are the brightness and resolution of the image formed of the screens. Image brightness is largely a function of the light output of the projector and the surface area of the screen, although other variables such as the characteristics of the screen can be important. Light output from a projector is usually specified in terms of lumens for digital projectors and in terms of lamp wattage for film projectors. Another aspect of brightness is the flatness of the illumination field (i.e., the reduction of brightness from the center of the image to the edges). Although an ideal projector would produce images that do not suffer from falloff of brightness at the edges, all projectors experience this problem to varying degrees.
Resolution is a dominant factor in determining the sharpness and clarity of the image on the screen. For film projection systems, resolution has traditionally been specified as the resolving power expressed as line pairs per millimeter (lp/mm). The resolving power of digital projection systems is usually expressed in terms of the number of pixels used to form the image, typically provided in two numbers, such as 800×600, where the first number refers to the number of pixels from across the width of the display area, and the second number refers to the number of pixels across the height of the display area. Both the brightness and perceived resolution of the image will decrease as the image size increases, and at some point the brightness or the resolution become unacceptable to viewers as the size of the screen increases.
One practical consideration for three-dimensional projection display systems is the cost of the projectors needed to produce images with adequate levels of brightness and resolution. The vast majority of projectors are manufactured in a few standard formats and are known generally as commodity projectors. Although it is possible to manufacture projectors with a custom format, such projectors are prohibitively expensive for most applications. In addition, the cost of commodity projectors increase markedly as the brightness and resolution capabilities are increased. For example, an LCD projector with a 3000 lumen output and native resolution of 1024×768 pixels will typically cost about three times as much as an LCD with a 1500 lumen output and native resolution of 800×600 pixels.
Another practical consideration is the aspect ratios of commodity projectors. The most common aspect ratio for digital projectors is the 4:3 ratio used in standard-format computer monitors, televisions, and digital audio-visual presentation systems. However, there is a strong trend towards wide-format projection systems such as the high-definition television (HDTV) aspect of 16:9. This presents a problem for three-dimensional projection systems project images onto screens with spherical, hemispherical, and like configurations, because such systems require the projection of a circular image onto a mirror and thus can only use the height of the display area. For example, a projection system using a projector with a resolution 1920×1080 pixels will only be able to project a circular image of 1080 pixels diameter, thereby wasting much of the display potential of the projector. In the context of flat screen systems, issues concerning brightness, resolution, and aspect ratio are often addressed by concurrently using multiple projectors. For example, it is possible to use two projectors together to superimpose identical images onto a common screen to increase the brightness of an image. Similarly, multiple projectors can be used to tile images onto a screen. In both cases, the images from the physically-separated projectors converge at the screen and the problem of adequately registering the images from the physically-separated projectors is addressed by means such as keystone correction (trapezoidal distortion) or lens shifting (physically moving a lens off the central optical axis). However, these approaches to using multiple projectors do not work well with three-dimensional projection systems featuring convex mirrors in the optical path because the images do not converge until they reach the convex mirror surface. Unless the images are combined prior to reaching a convex mirror, they will suffer from image distortion and registration problems that cannot be corrected with keystone correction or other means of compensating for distortion.
There is a need for a three-dimensional projection system that overcomes the problems described above, which the present invention addresses as discussed hereafter. It is an object of the present invention to provide an effective and economical means of combining images from multiple projectors in the context of three-dimensional projection systems.